python - functools.partial wants to use a positional argument as a keyword argument

Question

So I am trying to understand partial:

import functools

def f(x,y) :
    print x+y

g0 = functools.partial( f, 3 )
g0(1)

4 # Works as expected

In:

g1 = functools.partial( f, y=3 )
g1(1)

4 # Works as expected

In:

g2 = functools.partial( f, x=3 )
g2(1)

TypeError: f() got multiple values for keyword argument 'x'

The TypeError disappears if I use y as a keyword argument:

In:

g2( y=1 )

4

What causes the TypeError?

Answer 1

This has nothing to do with functools.partial, really. You are essentially calling your function like this:

f(1, x=3)

Python first fulfils the positional arguments, and your first argument is x. Then the keyword arguments are applied, and you again supplied x.

functools.partial() has no means to detect that you already supplied the first positional argument as a keyword argument instead. It will not augment your call by replacing the positional argument with a y= keyword argument.

When mixing positional and keyword arguments, you must take care not to use the same argument twice.